1. Field of the Invention
This invention relates in general to a method of radiation treatment of an article comprised of a sulfone polymer to relieve stress induced in the sulfone polymer or to stabilize the sulfone polymer against stress cracking. More particularly, the present invention relates to a method of rapidly relieving stress in an article comprised of a sulfone polymer or stabilizing such an article against stress cracking of the sulfone polymer in the manufacture of printed circuit boards.
2. Description of the Prior Art
It is well known that a variety of plastics may be electroplated, e.g., for the decorative arts by chemically conditioning them in strong oxidizing acids, e.g., chromic. Among the plastic materials that have been successfully plated are acrylonitrile-butadiene-styrene (ABS) copolymers, polyphenylene oxides (PPO), polysulfones, polyethersulfones, polycarbonates and nylon. Certain of these plastics cannot resist the temperature of soldering, i.e., about 260.degree. C. For example, ABS has a bond strength at room temperature of only 1 newton/mm and a softening temperature of 80.degree. C.-100.degree. C. A printed circuit board made of ABS, consequently cannot withstand soldering temperatures.
The use of thermoplastic polymers as substrate materials for printed wiring boards has had limited application due to marginal chemical compatibility of many low cost materials in the pretreatment solutions and plating baths associated with circuit board manufacture. In certain instances where the chemistry of the process is not deemed to be a degrading factor to the plastic, the stringent demands of component assembly and soldering as well as the post soldering flux removal cycles of solvent and/or detergent cleaning have ruled out many contending materials. A suitable thermoplastic should be compatible with circuit board chemistry; machinable on existing equipment; solderable and cleanable; and a suitable dielectric material.
Molded sulfone polymers have been used in very limited quantities as printed circuit base material, but only in high frequency applications where the low dielectric constant and dissipation factor of the polysulfone is required because of the manufacturing difficulties.
A comparison of electrical properties of sulfone polymers to other plastics is shown below:
______________________________________ DIE- DISSI- CONTIN- LECTRIC PATION UOUS CON- FAC- USE STANT TOR TEMP (10.sup.6 H.sub.z) (10.sup.6 H.sub.z) .degree.F. ______________________________________ ABS 2.4-3.8 .007-.015 180 PPO 2.6 .0007 220 PAPER REINFORCED 4.0 .018 250 EPOXY POLYCARBONATE 2.9 .010 250 GLASS REINFORCED 4.5 .020 290 POLYESTER GLASS CLOTH 4.5 .020 290 REINFORCED EPOXY POLYSULFONE 3.1 .003 345 POLYARYLSULFONE 3.7 .013 500 POLYETHERSULFONE 3.5 .006 395 POLYPHENYLSULFONE 3.45 .0076 395 GLASS REINFORCED 5.1 .017 425 EPOXY-POLYIMIDE GLASS REINFORCED 2.5 8 .times. 10.sup.-4 500 TEFLON ______________________________________
Extruded or molded sulfone polymer films, sheets or articles require special treatment to relieve stress crazing after being subjected to any mechanical operation. For example, drilling, machining, shearing, trimming, etc. can cause blistering or stress crazing of the sulfone polymer material. Presently, the stress relieving is accomplished by annealing at a temperature of about 167.degree. C. for at least 2 hours. Plating metals onto sulfone polymer also requires such an annealing step.
Printed circuit base materials consisting of sulfone polymer have not achieved wide usage because of the extreme processing difficulties and high price of the resin system. Annealing sulfone polymer base material for use as printed circuit substrates, for example, requires one or more annealing steps at a temperature of about 167.degree. C. to stress relieve the sulfone polymer for a minimum of 2-4 hours; 6-8 hours per annealing step is preferred. The sulfone polymer base is fixed between steel supporting plates to maintain its flatness and heated in an oven. The support plates are required to retain the flat shape of the polysulfone base since heating causes softening or flowing of the polysulfone material. The aforementioned laborious annealing steps are required two or more times during the manufacture of printed circuit boards. Failure to relieve stress can result in crazing and cracking of the polysulfone material in subsequent chemical processing or at a later time.